riscv: sifive_u: Update the plic hart config to support multicore
At present the PLIC is instantiated to support only one hart, while
the machine allows at most 4 harts to be created. When more than 1
hart is configured, PLIC needs to instantiated to support multicore,
otherwise an SMP OS does not work.
Signed-off-by: Bin Meng <bmeng.cn@gmail.com> Reviewed-by: Alistair Francis <alistair.francis@wdc.com> Signed-off-by: Palmer Dabbelt <palmer@sifive.com>